Effects of flow-field and mixture inhomogeneities on the ignition dynamics in continuous flow reactors

The objective of this study is to characterize effects of turbulence and flow-field inhomogeneities on the mixing and ignition-dynamics in flow reactors. Specific focus is on investigating the ignition characteristics of hydrogen-containing fuels at gas-turbine-relevant operating conditions. Two different model formulations are developed to describe the mixing, induction, and subsequent ignition and combustion. Utilizing these models, parametric studies are performed in a generic flow reactor configuration that is representative of commonly employed facilities. Diagnostics is developed to quantify the ignition dynamics. Results show that in the case of an initially homogeneous mixture, the ignition process is fairly insensitive to the underlying flow-field. However, by considering inhomogeneities in temperature and mixture composition it is shown that the ignition process exhibits a more pronounced sensitivity to temperature perturbations, and the ignition delay is only weakly sensitive to initial equivalence ratio perturbations. Simulation results show that temperature fluctuations of less than 10% of the mean temperature are sufficient to significantly affect the ignition-onset. Results from this parametric study identify the need for quantitative measurements of temperature and composition to better characterize flow reactor facilities. A time-scale analysis is performed to characterize competing physical processes that are associated with turbulent mixing, autoignition, and flame propagation. Qualitative comparisons with experimental data suggest the possibility for deflagrative ignition modes that can occur at low temperature operating conditions.